Method for joining a silicon plate to a second plate

ABSTRACT

A method is proposed for joining a silicon plate ( 1 ) to a second plate ( 2 ), a laser beam being directed through the silicon plate ( 1 ) at the second plate ( 2 ). In the process, the wavelength of the laser beam is selected in such a way that only a negligibly small amount of energy is absorbed in the silicon plate ( 1 ). A strongly absorbent material is hotmelted by the laser beam&#39;s energy and then produces a bond between the silicon plate ( 1 ) and the second plate ( 2 ).

BACKGROUND INFORMATION

[0001] The present invention is directed to a method for joining asilicon plate to a second plate according to the definition of thespecies in the independent claim. Methods are already known where asilicon plate is joined to a second plate by placing the silicon plateon the second plate. When the second plate is made of a specific glass,then, by increasing temperature and applying electric voltages, a bondcan be produced between silicon plate 1 and the second plate 2. Thisprocess is known to one skilled in the art as anodic bonding. Inaddition, silicon plates can be bonded to other plates using adhesiveprocesses.

SUMMARY OF THE INVENTION

[0002] In contrast, the method according to the present invention havingthe features of the independent claim has the advantage that by using alaser beam and by hotmelting a strongly absorbent material, especiallysmall join regions may be formed between a silicon plate and a secondplate. In this way, the space required for a connection of this kind maybe kept to a minimum. In addition, the method makes it possible toselect from a multiplicity of materials for the second plate, i.e., thesilicon plate may be joined to a second plate which is selected from amultiplicity of materials.

[0003] Advantageous further refinements and improvements are derivedfrom the features of the dependent claims. The strongly absorbentmaterial may be formed from either a thin, superficial layer, or,however, the second plate may be completely made of this material. Inthis way, a multiplicity of materials may be used for the heavilyabsorbent material, and a multiplicity of materials for the secondplate. The actual bonding may be carried out by using a plurality ofbonding methods, such as adhesive bonding, soldering,positive-engagement connections, or welding (heat sealing). Inparticular, by using a thin, superficial absorption layer, it is alsopossible to bond the silicon plate directly to a second silicon plate,which is especially advantageous in terms of the thermal expansioncoefficients. In particular, in this way, hermetically tight connectionsmay be formed where a cavity is then hermetically sealed by acircumferential seam. Within the circumferential seam, a contacting maythen be provided in the cavity. The method according to the presentinvention is especially suited for cases where a multiplicity ofstructures is produced simultaneously on the silicon plate or on thesecond plate. It is then advantageous, immediately after adjusting thetwo plates to one another, to fix this adjusted position by establishinga punctiform connection.

BRIEF DESCRIPTION OF THE DRAWING

[0004] Exemplary embodiments of the present invention are illustrated inthe drawings and explained in detail in the following description. Thefigures show:

[0005]FIGS. 1 and 2a first exemplary embodiment of the method accordingto the present invention;

[0006]FIG. 3 another exemplary embodiment of the method according to thepresent invention;

[0007]FIGS. 4 and 5 another exemplary embodiment of the method accordingto the present invention;

[0008]FIGS. 6, 7, and 8 another exemplary embodiment of the methodaccording to the present invention; and

[0009]FIG. 9 is yet another exemplary embodiment of the method accordingto the present invention, all the figures showing a cross-sectionthrough the plates.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0010]FIG. 1 shows a silicon plate 1 which is placed on a second plate2. A laser beam 3 is directed through silicon plate 1 at the surface ofsecond plate 2. The wavelength of laser beam 3 is selected in such a waythat only a negligibly small amount of energy is absorbed in siliconmaterial 1. This is achieved in that the wavelength of the laser lightis in the infrared range, since silicon is transparent in this frequencyrange. The material of second plate 2 is selected in such a way that anintense absorption already takes place in a thin, superficial layer.This intense absorption of the laser energy in a relatively thin layerresults in an intense thermal warming of this layer, which leads to ahotmelting of this region. The hotmelted region enables a bond to beproduced with silicon plate 1. In this context, various bondingmechanisms are conceivable. On the one hand, the hotmelted material ofsecond silicon plate 2 may be bonded to the surface of silicon wafer 1.This occurs, for example, when second plate 2 is made of a plasticmaterial which is hotmelted by the energy introduced by laser beam 3. Asuperficial bonding of such a plastic plate 2 to the surface of siliconwafer 1 then follows. In addition, a heat sealing may follow in such away that both second plate 2, as well as silicon plate 1, are melted bythe energy introduced by the laser beam. The energy absorbed in secondplate 2 is also transferred by thermal conduction to silicon wafer 1. Inthis context, a hotmelting of both second plate 2, as well as of siliconwafer 1 occurs. In the process, the melted material of silicon plate 1and of second plate 2 mix and form a blended melted mass which containsthe material of both silicon plate 1 as well as of second plate 2.Following the cooling, this molten region then forms the weldedconnection between silicon plate 1 and second plate 2. In addition, apositive engagement may be produced, as explained further below withreference to FIGS. 3 and 4. There may also be a soldered connectionbetween silicon plate 1 and second plate 2.

[0011]FIG. 2 depicts a bonding of silicon plate 1 to second plate 2 byway of a hotmelt-adhesive region 11. In this case, it is intended, inparticular, to use a plastic material for second plate 2. By introducingthe energy of laser beam 3, the plastic material of second plate 2 ishotmelted and, in the liquefied state, has wetted the surface of siliconwafer 1. Adhesive forces produce a bonding between silicon plate 1 andsecond plate 2.

[0012] As a further example, FIG. 3 shows the formation of the bondingregion as welded region 12. Starting out from FIG. 1, the energyintroduced by laser beam 3 hotmelts both second plate 2, as well as thematerial of silicon plate 1. The blending of the two materials in themolten state and the solidification following the cooling produce weldedregion 12. For second plate 2, it is intended, in particular, to useceramic material, glass or semiconductor materials (in particularsilicon) or metal. The material of second plate 2 is designed, in turn,to strongly absorb the energy from laser beam 3. In the case of silicon,this may be achieved by superficial layers (not shown here in greaterdetail) or by introducing suitable dopants. In this context, it isimportant that only a hotmelting of both the material of second plate 2,as well as of the material of silicon plate 1 occur, i.e., the twomaterials must be suitably adapted to one another in terms of theirmelting points. Moreover, the materials must be selected in a way thatresults in a thorough intermixing of the molten masses and a formationof a welded connection 12.

[0013]FIGS. 4 and 5 illustrate another method where the join region isformed by the energy introduced by laser beam 3 as a positive-engagementregion 13 (FIG. 5). To form positive-engagement region 13, silicon plate1 has a recess 14, and second plate 2 a lug 15. To produce theconnection, recess 14 is placed on lug 15, and the material of lug 15 isheated by the energy of laser beam 3. The hotmelting of the material oflug 15 results in a deformation of the lug; in particular, the moltenmaterial of lug 15 completely fills the cavity of recess 14. Recess 14should be designed, in particular, to have an undercut; i.e., thediameter of recess 14 should be greater depthwise than at the surfacewhere silicon plate 1 faces second plate 2. Undercuts of this kind maybe formed in silicon plate 1 by using etching processes. In this way, itis possible to produce a positive-engagement connection between asilicon plate 1 and a second plate 2.

[0014] In the description of the previous figures, the assumption wasmade that second plate 2 is completely made of one and the same materialwhich is strongly absorbent for the wavelength of laser beam 3. Forpractical applications, however, it completely suffices and isadvantageous in many cases when only a thin, superficial layer is madeof a strongly absorbent material, and when this strongly absorbent layeris situated between silicon plate 1 and second plate 2. In this context,it is also unimportant whether the strongly absorbent layer is placedbefore the bonding on silicon plate 1 or second plate 1. FIGS. 6-8 showa bonding process of this kind.

[0015] In FIG. 6, a silicon plate 1 and a second plate 2 are shown in apulled-apart state. On the side of silicon plate 1 that second plate 2faces, an absorption layer 20 and a recess 21 are provided. Amicromechanical structure 22 is situated on second plate 2, on the sidethat silicon plate 1 faces. As is apparent in FIG. 7, silicon plate 1having absorption layer 20 is placed on second plate 2, and a laser beamis then directed through silicon plate 1 at absorption layer 20. In theprocess of placing one plate on the other, recess 21 is positioned overmicromechanical structure 22. The dimensions of recess 21 are such thata cavity 23 remains above micromechanical structure 22. Laser beam 3introduces energy into absorption layer 20, thereby intensely heatingthis absorption layer. In the process sequence of FIGS. 6, 7 and 8, theassumption is made that the introduced energy is so powerful thatsilicon plate 1 and second plate 2 are melted in those regions which arein close proximity to the point of incidence of laser beam 3 onabsorption layer 20. The melted material of silicon plate 1, ofabsorption layer 20, and of second plate 2 mix in the molten state and,following laser irradiation and cooling, form a welded connection 12between silicon plate 1 and second plate 2. This state is shown incross-section in FIG. 8, this cross-section also illustrating a sectionthrough welded regions 12.

[0016] Various materials may be used for absorption layer 20. Forexample, for second plate 2, a glass plate or a silicon plate may beused, and absorption layer 20 may be made of plastic. By using a plasticlayer of this kind, an adhesive bond would then form, as was alreadydescribed with reference to FIG. 2. Absorption layer 20 may also beprovided with one lug 15, and silicon plate 1 may also have recesses 14.Then, similarly to the manner already described with reference to FIGS.4 and 5, a bond could form using a positive-engagement region 13. Forthis form as well, an absorption layer 20 may then be used, which isrelatively thin in comparison to plates 1 and 2 and which is equippedwith appropriate lugs 15.

[0017] The formation of a welded connection is described with referenceto FIGS. 6-7. This method is applicable, for example, when second plate2 is likewise made of a silicon wafer. For the absorbent layer, thinmetal layers, for example of aluminum, aluminum-silicon copper,platinum, titanium, chromium or other refractory metals, may be used. Inaddition, germanium, silicon-germanium or highly doped polysiliconlayers may be used for absorption layer 20. As other materials forabsorption layer 20, oxides and nitrides, for example silicon oxide andsilicon nitride, may be used as absorption layers. The absorption layermay be used in relatively small layer thicknesses, it being recommended,however, that the layer thickness correspond approximately to thepenetration depth of the laser, i.e., the reciprocal value of theabsorption coefficient. In this context, depending on the material used,typical layer thicknesses are on the order of more than 100 nanometers.The material for layers of this kind may be processed using conventionalthin-film techniques, such as sputtering, vapor deposition, spin-ondeposition, CVD deposition, epitaxy, and the like. In addition,absorption layers 20 may be structurally formed, i.e., they are placedonly where weld connections 12 are to be produced later. To the extentthat it is simpler in terms of process technology, and there is no needto fear that the layers will adversely affect any existingmicromechanical structures 22, these layers may also be deposited overthe entire surface.

[0018] Besides the bonds already described using adhesive bonding,positive engagement, and welding (heat sealing), the bond formed byabsorption layer 20 may also be produced by soldering. In this context,absorption layer 20 is made of a material which, in the hotmelted state,produces a soldered connection between silicon plate 1 and second plate2. This may be especially useful when second plate 2 is a metal plate.

[0019] FIGS. 6-8 show that a micromechanical structure 22 is placed in acavity 23 which is formed by recess 21. A process of this kind has to dowith the packaging of a micromechanical component, where micromechanicalcomponent 22 is packaged in a hermetically tight fashion in a cavity 23.In this context, welded connection 12 is formed as a circumferentialseam, i.e., welded connection 12 completely surrounds micromechanicalstructure 22 in the plane formed by the two plates 1, 2. The problemarises then of how this micromechanical structure 22 is electricallycontacted. In this regard, reference is made to FIG. 9 which depicts across-section through an exemplary micromechanical structure. Incontrast to the previously described figures, here the silicon plate ispositioned as the bottommost plate, and the laser beams are radiatedfrom underneath, as shown by the arrows. Welded regions 12 are formedhere by laser beams 3 and completely surround micromechanical structure22. Micromechanical structures 22 are also shown merely schematicallyhere. On the top side, micromechanical structures 22 are covered byanother cover plate 50, which, by way of a spacer layer 51, are spacedapart from micromechanical structures 22. Micromechanical structures 22are formed by introducing trenches into second plate 2 which splitsecond plate 2 from top to bottom. Trench structures of this kind areable to be introduced quite simply into silicon plates, so that secondplate 2 in this case is usually made of silicon. In an edge region, themicromechanical structures have a join region 52, which is separated bya trench structure 53 from the remaining material of silicon plate 2. Inthis join region 52, the material of silicon plate 2 directly bonds withthe material of cover plate 50. For cover plate 50, one likewiseenvisages a silicon plate which is designed to be conductive, at leastin some regions, by doping. Above join region 52, a contact region 54 isformed on which a contact metallization 55 is applied. Contact region54, in turn, is electrically insulated by trenches 53 from the rest ofsilicon plate 50. By way of contact metallization 55 and subjacentcontact region 54 or join region 52, micromechanical structures 22located in cavity 23 may be electrically contacted. At the boundarysurface between silicon plates 1, 2, no conductor track is able to beformed, namely, since a conductor track of this kind would be destroyedby circumferential welded connection 12. For that reason, it isnecessary, within this circumferential seam, to provide an electricalfeed-through lead via join region 52 or contact region 54 through whichmicromechanical structure 22 is contacted in cavity 23.

[0020] The method according to the present invention is preferablyapplied to silicon plates which have a multiplicity of structures. Sincesilicon plates are transparent to infrared light, by way of siliconplate 1, it is possible to align silicon plate 1 to second plate 2. Aspot-type bonding of the two plates may be achieved using laserradiation, which effectively prevents the two plates from slippingrelatively to one another in the further processing. The actual bondsmay be subsequently formed. The processing duration, for example, may belonger than the time required to adjust the two plates 1, 2 relativelyto one another, or to perform the spot bonding. This is of particularinterest when a multiplicity of structures are formed in silicon plate 1or in second plate 2 and when it is intended to produce connections overlarge regions. This is particularly the case when a multiplicity ofindividual structures are provided, which are each to be hermeticallypackaged, so that a circumferential connecting seam is required aroundeach structure. This is the case, for example, when micromechanicalstructures are hermetically sealed, since each of these micromechanicalstructures 22 is surrounded by a complete connecting seam.

What is claimed is:
 1. A method for joining a silicon plate (1) to asecond plate (2), the silicon plate (1) and the second plate (2) beingplace one upon the other for purposes of joining the two plates, whereina laser beam (3) is directed through the silicon plate (1) at the secondplate; the wavelength of the laser beam being selected in such a waythat only a small amount of energy is absorbed in the silicon plate (1);on a surface of the silicon plate (1) facing the second plate (2),strongly absorbent material is provided, which almost completely absorbsthe energy of the laser beam and is hotmelted by the energy introducedby the laser beam (3); the melted material is cooled again, and a bondis produced between the silicon plate (1) and the second plate (2). 2.The method as recited in claim 1, wherein a layer (20) of the stronglyabsorbent material is situated in the area of a bonding site between thesilicon plate (1) and the second plate (2).
 3. The method as recited inclaim 1, wherein the second plate (2) is completely formed from thestrongly absorbent material.
 4. The method as recited in one of thepreceding claims, wherein silicon, metal, glass, plastic, metal,germanium or silicon germanium are selected as material for the secondplate (2).
 5. The method as recited in one of the preceding claims,wherein metal, germanium, silicon germanium, heavily doped polysiliconor a plastic is used as strongly absorbent material.
 6. The method asrecited in one of the preceding claims, wherein the hotmelted, stronglyabsorbent material produces a hotmelt-adhesive connection, a solderedconnection, a positive-engagement connection, or a welded connectionbetween the silicon plate (1) and the second plate (2).
 7. The method asrecited in one of the preceding claims, wherein the bond is formed usinga circumferential seam which completely surrounds an area of the surfaceof the silicon plate; an electrical feed-through lead (52, 54) beingprovided within the circumferential seam in the silicon plate (1) or thesecond plate (2), through which an electrical contact to an interiorspace (23) is established which is situated between the silicon plate(1) and the second plate (2).
 8. The method as recited in one of thepreceding claims, wherein a multiplicity of structures is provided onthe silicon plate (1) and the second plate (2); the structures areadjusted relatively to one another; and the adjustment is accomplishedby directing infrared light through the silicon plate (1).
 9. The methodas recited in claim 8, wherein following the adjustment, spot-typeconnections are formed between the silicon plate (1) and the secondplate (2), through which the adjusted position of the silicon plate (1)is retained relatively to the second plate (2) in the subsequent bondingprocess.